There are numerous industrial processes in which fluid streams are generated containing suspended solids which ultimately require separation and classification of the solids from the suspending liquid. An example of such an industrial process is the Fischer-Tropsch three-phase slurry process for the synthesis of hydrocarbons.
Typically, a Fischer-Tropsch three-phase slurry, hydrocarbon synthesis process is conducted in a bubble column reactor by contacting a stream of synthesis gas (comprising H2 and CO) with a liquid suspension of solid catalyst. The synthesis gas will have an H2:CO molar ratio of from about 1:1 to about 3:1. The dispersing liquid is primarily linear hydrocarbon reaction product. To facilitate contact between catalyst and the synthesis gas, the gas is fed into the bottom of the bubble column through a gas distributor that produces small gas bubbles. As the synthesis gas bubbles rise through the column, they not only disperse the catalyst in the liquid, but they also react to form hydrocarbon products that are mainly liquids under the reaction temperature and pressure conditions. Any gaseous products that are formed rise to the top of the reactor from which they are removed.
Because it is necessary to maintain the slurry in the reactor at a constant level, liquid products are continuously or periodically removed from the reactor. In doing so, however, it is important to separate dispersed catalyst particles from the liquid being removed to maintain a constant inventory of catalyst in the reactor. Generally, the separation is conducted in a filtration zone located in the slurry bed. The filtration zone typically comprises cylindrical filtering media through which liquid passes from the exterior to the interior of the filtering media where it is collected and removed from the reactor. In some reactor designs, liquid product is filtered in an external filtration system, and separated catalyst is returned to the reactor.
Over time, the hydrodynamic conditions existing in the bubble column result in some attrition of the catalyst, thereby forming catalyst particles known as “fines”. Fines are those particles having a size less than 10 microns. In contrast, coarse catalyst particles are those particles having a size greater than or equal to 10 microns. The size of the openings in the filtering media will, of course, determine the amount of fines likely to pass through the filter with the liquid product. Thus, the liquid product removed from the bubble column may be subjected to a second stage separation process to provide a substantially fine-free liquid product and a second stage solids-containing stream requiring further liquids-solids separation. The volume and solids content of the second stage stream will vary depending upon the extent of catalyst attrition. Also, over time, fines not passing through the filter will begin to plug the filter. Consequently, filter efficiency is decreased, and remedial action such as backwashing with a liquid stream becomes necessary. Backwashing the filters typically is conducted periodically in a pulsing mode with a liquid. This results in an undesirable accumulation of fines in the slurry. Consequently, it is necessary to control the amount of fines that accumulates in the slurry. Thus, a slip stream containing fines may be removed when necessary, and this fines-laden slip stream may be subjected to separation and classification.
As is known in the art, the catalyst used in a three-phase slurry reactor is regenerated either continuously or periodically. Typically, the catalyst is regenerated when its activity decreases to the point where reactant conversion cannot be maintained. In the regeneration process, the first step generally requires the removal of wax from the catalyst. In one method, this is accomplished by washing the catalyst with a solvent. The solvent typically is separated from the catalyst by decanting, thereby providing a liquid stream containing solids, primarily fines. These solids also require classification and separation from the liquid.
Obviously, the rate of flow of catalyst-containing stream used to control the slurry fines concentration will be different from that generated in the regeneration step as will their solids content and particle size distribution. So too will it be different from the solids-containing stream generated in the above mentioned second stage separation process.
Other sources of solids-containing liquid streams requiring solids classification and separation are those obtained from occasional flushing of the bubble column reactor cone, unit purges, process equipment flushes, etc. These, of course, will be generated at different times, at varying rates, and will have different solids content.
An object of this present invention is to provide a method and system for classifying and separating solids suspended in liquid streams that are generated at varying rates and which contain differing amounts of solids.
Another object of the invention is to provide an integrated system for particle size classification and for solids thickening, which system is capable of functioning under differing load conditions.
These and other objects of the invention will become apparent from the description of the invention below.